Digital holographic microscopy real-time monitoring of cytoarchitectural alterations during simulated microgravity

Lab-on-a-Chip Technologies for Microgravity Simulation and Space Applications

Gravity plays an important role in the development of life on earth. The effect of gravity on living organisms can be investigated by controlling the magnitude of gravity. Most reduced gravity experiments are conducted on the Lower Earth Orbit (LEO) in the International Space Station (ISS). However, running experiments in ISS face challenges such as high cost, extreme condition, lack of direct accessibility, and long waiting period. Therefore, researchers have developed various ground-based devices and methods to perform reduced gravity experiments. However, the advantage of space conditions for developing new drugs, vaccines, and chemical applications requires more attention and new research. Advancements in conventional methods and the development of new methods are necessary to fulfil these demands. The advantages of Lab-on-a-Chip (LOC) devices make them an attractive option for simulating microgravity. This paper briefly reviews the advancement of LOC technologies for simulating microgravity in an earth-based laboratory.

Living Sample Viability Measurement Methods from Traditional Assays to Nanomotion

Living sample viability measurement is an extremely common process in medical, pharmaceutical, and biological fields, especially drug pharmacology and toxicology detection. Nowadays, there are a number of chemical, optical, and mechanical methods that have been developed in response to the growing demand for simple, rapid, accurate, and reliable real-time living sample viability assessment. In parallel, the development trend of viability measurement methods (VMMs) has increasingly shifted from traditional assays towards the innovative atomic force microscope (AFM) oscillating sensor method (referred to as nanomotion), which takes advantage of the adhesion of living samples to an oscillating surface. Herein, we provide a comprehensive review of the common VMMs, laying emphasis on their benefits and drawbacks, as well as evaluating the potential utility of VMMs. In addition, we discuss the nanomotion technique, focusing on its applications, sample attachment protocols, and result display methods. Furthermore, the challenges and future perspectives on nanomotion are commented on, mainly emphasizing scientific restrictions and development orientations.

Effects of Titanium Dioxide Nanoparticles on Cell Growth and Migration of A549 Cells under Simulated Microgravity

With the increasing application of nanomaterials in aerospace technology, the long-term space exposure to nanomaterials especially in the space full of radiation coupled with microgravity condition has aroused great health concerns of the astronauts. However, few studies have been conducted to assess these effects, which are crucial for seeking the possible intervention strategy. Herein, using a random positioning machine (RPM) to simulate microgravity, we investigated the behaviors of cells under simulated microgravity and also evaluated the possible toxicity of titanium dioxide nanoparticles (TiO₂ NPs), a multifunctional nanomaterial with potential application in aerospace. Pulmonary epithelial cells A549 were exposed to normal gravity (1 g) and simulated gravity (~10⁻³ g), respectively. The results showed that simulated microgravity had no significant effect on the viability of A549 cells as compared with normal gravity within 48 h. The effects of TiO₂ NPs exposure on cell viability and apoptosis were marginal with only a slightly decrease in cell viability and a subtle increase in apoptosis rate observed at a high concentration of TiO₂ NPs (100 μg/mL). However, it was observed that the exposure to simulated microgravity could obviously reduce A549 cell migration compared with normal gravity. The disruption of F-actin network and the deactivation of FAK (Tyr397) might be responsible for the impaired mobility of simulated microgravity-exposed A549 cells. TiO₂ NPs exposure inhibited cell migration under two different gravity conditions, but to different degrees, with a milder inhibition under simulated microgravity. Meanwhile, it was found that A549 cells internalized more TiO₂ NPs under normal gravity than simulated microgravity, which may account for the lower cytotoxicity and the lighter inhibition of cell migration induced by the same exposure concentration of TiO₂ NPs under simulated microgravity at least partially. Our study has provided some tentative information on the effects of TiO₂ NPs exposure on cell behaviors under simulated microgravity.

Adaptation and Changes in Actin Dynamics and Cell Motility as Early Responses of Cultured Mammalian Cells to Altered Gravitational Vector

Cultured mammalian cells have been shown to respond to microgravity (μG), but the molecular mechanism is still unknown. The study we report here is focused on molecular and cellular events that occur within a short period of time, which may be related to gravity sensing by cells. Our assumption is that the gravity-sensing mechanism is activated as soon as cells are exposed to any new gravitational environment. To study the molecular events, we exposed cells to simulated μG (SμG) for 15 min, 30 min, 1 h, 2 h, 4 h, and 8 h using a three-dimensional clinostat and made cell lysates, which were then analyzed by reverse phase protein arrays (RPPAs) using a panel of 453 different antibodies. By comparing the RPPA data from cells cultured at 1G with those of cells under SμG, we identified a total of 35 proteomic changes in the SμG samples and found that 20 of these changes took place, mostly transiently, within 30 min. In the 4 h and 8 h samples, there were only two RPPA changes, suggesting that the physiology of these cells is practically indistinguishable from that of cells cultured at 1 G. Among the proteins involved in the early proteomic changes were those that regulate cell motility and cytoskeletal organization. To see whether changes in gravitational environment indeed activate cell motility, we flipped the culture dish upside down (directional change in gravity vector) and studied cell migration and actin cytoskeletal organization. We found that compared with cells grown right-side up, upside-down cells transiently lost stress fibers and rapidly developed lamellipodia, which was supported by increased activity of Ras-related C3 botulinum toxin substrate 1 (Rac1). The upside-down cells also increased their migratory activity. It is possible that these early molecular and cellular events play roles in gravity sensing by mammalian cells. Our study also indicated that these early responses are transient, suggesting that cells appear to adapt physiologically to a new gravitational environment.

Module to Support Real-Time Microscopic Imaging of Living Organisms on Ground-Based Microgravity Analogs

Since opportunities for spaceflight experiments are scarce, ground-based microgravity simulation devices (MSDs) offer accessible and economical alternatives for gravitational biology studies. Among the MSDs, the random positioning machine (RPM) provides simulated microgravity conditions on the ground by randomizing rotating biological samples in two axes to distribute the Earth’s gravity vector in all directions over time. Real-time microscopy and image acquisition during microgravity simulation are of particular interest to enable the study of how basic cell functions, such as division, migration, and proliferation, progress under altered gravity conditions. However, these capabilities have been difficult to implement due to the constantly moving frames of the RPM as well as mechanical noise. Therefore, we developed an image acquisition module that can be mounted on an RPM to capture live images over time while the specimen is in the simulated microgravity (SMG) environment. This module integrates a digital microscope with a magnification range of 20× to 700×, a high-speed data transmission adaptor for the wireless streaming of time-lapse images, and a backlight illuminator to view the sample under brightfield and darkfield modes. With this module, we successfully demonstrated the real-time imaging of human cells cultured on an RPM in brightfield, lasting up to 80 h, and also visualized them in green fluorescent channel. This module was successful in monitoring cell morphology and in quantifying the rate of cell division, cell migration, and wound healing in SMG. It can be easily modified to study the response of other biological specimens to SMG.

The influence of spaceflight and simulated microgravity on bacterial motility and chemotaxis

As interest in space exploration rises, there is a growing need to quantify the impact of microgravity on the growth, survival, and adaptation of microorganisms, including those responsible for astronaut illness. Motility is a key microbial behavior that plays important roles in nutrient assimilation, tissue localization and invasion, pathogenicity, biofilm formation, and ultimately survival. Very few studies have specifically looked at the effects of microgravity on the phenotypes of microbial motility. However, genomic and transcriptomic studies give a broad general picture of overall gene expression that can be used to predict motility phenotypes based upon selected genes, such as those responsible for flagellar synthesis and function and/or taxis. In this review, we focus on specific strains of Gram-negative bacteria that have been the most studied in this context. We begin with a discussion of Earth-based microgravity simulation systems and how they may affect the genes and phenotypes of interest. We then summarize results from both Earth- and space-based systems showing effects of microgravity on motility-related genes and phenotypes.

Scalable Microgravity Simulator Used for Long-Term Musculoskeletal Cells and Tissue Engineering

We introduce a new benchtop microgravity simulator (MGS) that is scalable and easy to use. Its working principle is similar to that of random positioning machines (RPM), commonly used in research laboratories and regarded as one of the gold standards for simulating microgravity. The improvement of the MGS concerns mainly the algorithms controlling the movements of the samples and the design that, for the first time, guarantees equal treatment of all the culture flasks undergoing simulated microgravity. Qualification and validation tests of the new device were conducted with human bone marrow stem cells (bMSC) and mouse skeletal muscle myoblasts (C2C12). bMSC were cultured for 4 days on the MGS and the RPM in parallel. In the presence of osteogenic medium, an overexpression of osteogenic markers was detected in the samples from both devices. Similarly, C2C12 cells were maintained for 4 days on the MGS and the rotating wall vessel (RWV) device, another widely used microgravity simulator. Significant downregulation of myogenesis markers was observed in gravitationally unloaded cells. Therefore, similar results can be obtained regardless of the used simulated microgravity devices, namely MGS, RPM, or RWV. The newly developed MGS device thus offers easy and reliable long-term cell culture possibilities under simulated microgravity conditions. Currently, upgrades are in progress to allow real-time monitoring of the culture media and liquids exchange while running. This is of particular interest for long-term cultivation, needed for tissue engineering applications. Tissue grown under real or simulated microgravity has specific features, such as growth in three-dimensions (3D). Growth in weightlessness conditions fosters mechanical, structural, and chemical interactions between cells and the extracellular matrix in any direction.

Quantitative observations on cytoskeleton changes of osteocytes at different cell parts using digital holographic microscopy

Cytoskeletons such as F-actin have different distributions in different cell parts and they are the cause of different degrees of cell collapse when the F-actin is disrupted. It is challenging to use conventional methods such as fluorescence microscopy and atomic force microscopy to conduct real-time and three-dimensional observations on the dynamic processes at different cell parts due to the slow measuring speed and the need for live-cell staining. In this study, the morphological variations of different bone cell parts caused by F-actin disruption are dynamically measured by using digital holographic microscopy (DHM). We separately analyze local parameters (cell height and cell width) and global parameters (cell projected area and cell volume) of cells to address variations of specific cell areas and quantify the changing process of the whole cell. We found significant differences in temporal variations of both local and global cell parameters between the cell body and cell process, which is consistent with the qualitative observation by fluorescence staining. Our study not only validates the unique ability of DHM to simultaneously investigate the dynamic process at different cell parts, but also provides sufficient experimental bases for exploring the mechanism for F-actin disruption.

Time-averaged simulated microgravity (taSMG) inhibits proliferation of lymphoma cells, L-540 and HDLM-2, using a 3D clinostat

Background

Gravity is omnipresent on Earth; however, humans in space, such as astronauts at the International Space Station, experience microgravity. Long-term exposure to microgravity is considered to elicit physiological changes, such as muscle atrophy, in the human body. In addition, certain types of cancer cells demonstrate inhibited proliferation under condition of time-averaged simulated microgravity (taSMG). However, the response of human Hodgkin’s lymphoma cancer cells to reduced gravity, and the associated physiological changes in these cells, have not been elucidated.

Methods

In this study, the proliferation of human Hodgkin’s lymphoma cancer cells (L-540 and HDLM-2) under taSMG condition (<10⁻³ G, 1 G is defined as 9.8 m/s²) was studied using a 3D clinostat. Normal human dermal fibroblast (HDF) was proliferated in the same condition as a control group. For the development of 3D clinostat, two motors were used to actuate the frames. Electrical wires for power supply and communication were connected via slip ring. For symmetrical path of gravitational vector, optimal angular velocities of the motors were found using simulation results. Under the condition of taSMG implemented by the 3D clinostat, proliferation of the cells was observed for 3 days.

Results

The results indicated that proliferation of these cancer cells was significantly (p < 0.0005) inhibited under taSMG, whereas proliferation of normal HDF cells was not affected.

Conclusions

Findings in this study could be significantly valuable in developing novel strategies for selective killing of cancer cells such as lymphoma.

Accurate quantitative phase digital holographic microscopy with single- and multiple-wavelength telecentric and nontelecentric configurations

In this work, we investigate, both theoretically and experimentally, single-wavelength and multiwavelength digital holographic microscopy (DHM) using telecentric and nontelecentric configurations in transmission and reflection modes. A single-wavelength telecentric imaging system in DHM was originally proposed to circumvent the residual parabolic phase distortion due to the microscope objective (MO) in standard nontelecentric DHM configurations. However, telecentric configurations cannot compensate for higher order phase aberrations. As an extension to the telecentric and nontelecentric arrangements in single-wavelength DHM (SW-DHM), we propose multiple-wavelength telecentric DHM (MW-TDHM) in reflection and transmission modes. The advantages of MW-TDHM configurations are to extend the vertical measurement range without phase ambiguity and optically remove the parabolic phase distortion caused by the MO in traditional MW-DHM. These configurations eliminate the need for a second reference hologram to subtract the two-phase maps and make digital automatic aberration compensation easier to apply compared to nontelecentric configurations. We also discuss a reconstruction algorithm that eliminates the zero-order and virtual images using spatial filtering and another algorithm that minimizes the intensity of fluctuations using apodization. In addition, we employ two polynomial models using 2D surface fitting to compensate digitally for chromatic aberration (in the multiwavelength case) and for higher order phase aberrations. A custom-developed user-friendly graphical user interface is employed to automate the reconstruction processes for all configurations. Finally, TDHM is used to visualize cells from the highly invasive MDA-MB-231 cultured breast cancer cells.

RhoGTPases as key players in mammalian cell adaptation to microgravity

A growing number of studies are revealing that cells reorganize their cytoskeleton when exposed to conditions of microgravity. Most, if not all, of the structural changes observed on flown cells can be explained by modulation of RhoGTPases, which are mechanosensitive switches responsible for cytoskeletal dynamics control. This review identifies general principles defining cell sensitivity to gravitational stresses. We discuss what is known about changes in cell shape, nucleus, and focal adhesions and try to establish the relationship with specific RhoGTPase activities. We conclude by considering the potential relevance of live imaging of RhoGTPase activity or cytoskeletal structures in order to enhance our understanding of cell adaptation to microgravity-related conditions.

Simulated microgravity: critical review on the use of random positioning machines for mammalian cell culture

Random Positioning Machines (RPMs) have been used since many years as a ground-based model to simulate microgravity. In this review we discuss several aspects of the RPM. Recent technological development has expanded the operative range of the RPM substantially. New possibilities of live cell imaging and partial gravity simulations, for example, are of particular interest. For obtaining valuable and reliable results from RPM experiments, the appropriate use of the RPM is of utmost importance. The simulation of microgravity requires that the RPM's rotation is faster than the biological process under study, but not so fast that undesired side effects appear. It remains a legitimate question, however, whether the RPM can accurately and reliably simulate microgravity conditions comparable to real microgravity in space. We attempt to answer this question by mathematically analyzing the forces working on the samples while they are mounted on the operating RPM and by comparing data obtained under real microgravity in space and simulated microgravity on the RPM. In conclusion and after taking the mentioned constraints into consideration, we are convinced that simulated microgravity experiments on the RPM are a valid alternative for conducting examinations on the influence of the force of gravity in a fast and straightforward approach.

The role of the cytoskeleton in sensing changes in gravity by nonspecialized cells

A large body of evidence indicates that single cells in vitro respond to changes in gravity, and that this response might play an important role for physiological changes at the organism level during spaceflight. Gravity can lead to changes in cell proliferation, differentiation, signaling, and gene expression. At first glance, gravitational forces seem too small to affect bodies with the size of a cell. Thus, the initial response to gravity is both puzzling and important for understanding physiological changes in space. This also offers a unique environment to study the mechanical response of cells. In the past 2 decades, important steps have been made in the field of mechanobiology, and we use these advances to reevaluate the response of single cells to changes in gravity. Recent studies have focused on the cytoskeleton as initial gravity sensor. Thus, we review the observed changes in the cytoskeleton in a microgravity environment, both during spaceflight and in ground-based simulation techniques. We also evaluate to what degree the current experimental evidence supports the cytoskeleton as primary gravity sensor. Finally, we consider how the cytoskeleton itself could be affected by changed gravity. To make the next step toward understanding the response of cells to altered gravity, the challenge will be to track changes quantitatively and on short timescales.

Quantitative phase imaging techniques for the study of cell pathophysiology: from principles to applications

A cellular-level study of the pathophysiology is crucial for understanding the mechanisms behind human diseases. Recent advances in quantitative phase imaging (QPI) techniques show promises for the cellular-level understanding of the pathophysiology of diseases. To provide important insight on how the QPI techniques potentially improve the study of cell pathophysiology, here we present the principles of QPI and highlight some of the recent applications of QPI ranging from cell homeostasis to infectious diseases and cancer.

Digital holographic microscopy long-term and real-time monitoring of cell division and changes under simulated zero gravity

The long-term and real-time monitoring the cell division and changes of osteoblasts under simulated zero gravity condition were succeed by combing a digital holographic microscopy (DHM) with a superconducting magnet (SM). The SM could generate different magnetic force fields in a cylindrical cavity, where the gravitational force of biological samples could be canceled at a special gravity position by a high magnetic force. Therefore the specimens were levitated and in a simulated zero gravity environment. The DHM was modified to fit with SM by using single mode optical fibers and a vertically-configured jig designed to hold specimens and integrate optical device in the magnet's bore. The results presented the first-phase images of living cells undergoing dynamic divisions and changes under simulated zero gravity environment for a period of 10 hours. The experiments demonstrated that the SM-compatible DHM setup could provide a highly efficient and versatile method for research on the effects of microgravity on biological samples.

Enhanced robustness digital holographic microscopy for demanding environment of space biology

We describe an optimized digital holographic microscopy system (DHM) suitable for high-resolution visualization of living cells under conditions of altered macroscopic mechanical forces such as those that arise from changes in gravitational force. Experiments were performed on both a ground-based microgravity simulation platform known as the random positioning machine (RPM) as well as during a parabolic flight campaign (PFC). Under these conditions the DHM system proved to be robust and reliable. In addition, the stability of the system during disturbances in gravitational force was further enhanced by implementing post-processing algorithms that best exploit the intrinsic advantages of DHM for hologram autofocusing and subsequent image registration. Preliminary results obtained in the form of series of phase images point towards sensible changes of cytoarchitecture under states of altered gravity.

Early cell death detection with digital holographic microscopy

BACKGROUND

Digital holography provides a non-invasive measurement of the quantitative phase shifts induced by cells in culture, which can be related to cell volume changes. It has been shown previously that regulation of cell volume, in particular as it relates to ionic homeostasis, is crucially involved in the activation/inactivation of the cell death processes. We thus present here an application of digital holographic microscopy (DHM) dedicated to early and label-free detection of cell death.

METHODS AND FINDINGS

We provide quantitative measurements of phase signal obtained on mouse cortical neurons, and caused by early neuronal cell volume regulation triggered by excitotoxic concentrations of L-glutamate. We show that the efficiency of this early regulation of cell volume detected by DHM, is correlated with the occurrence of subsequent neuronal death assessed with the widely accepted trypan blue method for detection of cell viability.

CONCLUSIONS

The determination of the phase signal by DHM provides a simple and rapid optical method for the early detection of cell death.